Transformer cores for large power transformers are assembled by arranging a plurality of core steel laminations in a core bottom yoke and core leg arrangement in the horizontal plane. When the three-phase core is to be assembled, the three legs and the bottom yoke that interconnects the three legs are assembled in a stack of core steel laminations that are bound together by any of several means, for instance, by the use of an epoxy impregnated glass tape. The assembled core legs and bottom core yoke are then raised to a vertical position for ease in assembling the transformer windings around each of the core legs. Once the transformer windings are in place, the transformer core is completed by inserting the upper yoke core steel laminations and fastening the laminations together by means of a clamping structure. The placement of a plurality of core steel laminations upon the partially completed core assembly is a difficult and time consuming task to be performed in the latter stages of transformer manufacture. It is therefore desirable to be able to limit the amount of time required to assemble the core yoke laminations at a time in the transformer manufacturing process when the transformer is near completion. The purpose of this invention is to provide a method and means for assembling the upper core yoke as a single unitary assembly at a substantial saving in assembly time and cost.